This invention relates to a method of preparing trifluoromethanesulfonic acid anhydride (CF.sub.3 SO.sub.2).sub.2 O by the dehydrating reaction of trifluoromethanesulfonic acid with phosphorus pentaoxide, and more particularly to a method of industrially preparing the acid anhydride with efficient recovery of unreacted trifluoromethanesulfonic acid.
Trifluoromethanesulfonic acid anhydride is a compound useful as a raw material for the synthesis of medicines, polymerization initiators, etc.
Trifluoromethanesulfonic acid anhydride is prepared usually and almost exclusively by the dehydrating reaction of trifluoromethanesulfonic acid with phosphorus pentaoxide. However, the rate of this reaction becomes extremely low as the conversion of the acid into the acid anhydride reaches about 60%, and hence in industrial practice the dehydrating reaction is terminated at the stage of about 60% conversion. As a natural consequence about 40% of trifluoromethanesulfonic acid subjected reaction remains unreacted in the residue of the reaction system. From an industrial point of view it is necessary to recover and reuse the unreacted acid. Actually, however, the recovery is difficult. First, it is not easy to extract the residue from the reactor since a major portion of the residue is phosphorus pentaoxide in solid form. Further, it is impossible to efficiently recover the acid by subjecting the residue to reduced pressure distillation since the acid in the residue is absorbed in the oxide.